The challenge, however, with today’s converged
networks is that multiple applications are contending for bandwidth, and some
applications such as, voice can be more intolerant of delay (that is, latency)
than other applications such as, an FTP file transfer. A lack of bandwidth is
the overshadowing issue for most quality problems.

When a lack of
bandwidth exists, packets can suffer from one or more of the following
symptoms:

Delay—Delay is the time that is required for a packet to
travel from its source to its destination. You might witness delay on the
evening news, when the news anchor is talking through satellite to a foreign
news correspondent. Because of the satellite delay, the conversation begins to
feel unnatural.

Jitter—Jitter is the uneven arrival of packets. For example,
consider that in a Voice over IP (VoIP) conversation, packet 1 arrives. Then, 20
ms later, packet 2 arrives. After another 70 ms, packet 3 arrives, and then
packet 4 arrives 20 ms behind packet 3. This variation in arrival times (that
is, variable delay) is not dropping packets, but this jitter can be interpreted
by the listener as dropped packets.

Drops—Packet drops occur when a link is congested and a
buffer overflows. Some types of traffic, such as User Datagram Protocol (UDP)
traffic (for example, voice), are not retransmitted if packets are
dropped.

Fortunately, quality of service (QoS) features that are available on Cisco
routers and switches can recognize your “important” traffic and then
treat that traffic in a special way. For example, you might want to allocate 128
kbps of bandwidth for your VoIP traffic and also give that traffic priority
treatment.

Consider water that is flowing through a series of pipes with
varying diameters. The water’s flow rate through those pipes is limited to
the water’s flow rate through the pipe with the smallest diameter.
Similarly, as a packet travels from its source to its destination, its effective
bandwidth is the bandwidth of the slowest link along that path.

Because your primary challenge is a lack of bandwidth, the logical question
is, “How do you increase available bandwidth?” A knee-jerk response
to that question is often, “Add more bandwidth.” Although adding
more bandwidth is the best solution, it comes at a relatively high
cost.

Compare your network to a highway system in a large city. During
rush hour, the lanes of the highway are congested, but the lanes can be
underutilized during other periods of the day. Instead of just building more
lanes to accommodate peak traffic rates, the highway engineers add carpool
lanes. Cars with two or more riders can use the reserved carpool lane. These
cars have a higher priority on the highway. Similarly, you can use QoS features
to give your mission-critical applications higher-priority treatment in times of
network congestion.

Some of the QoS features that can address issues of
delay, jitter, and packet loss include the following:

Queuing—Queuing can send higher-priority traffic ahead of
lower-priority traffic and make specific amounts of bandwidth available for
those traffic types. Examples of queuing strategies that you consider later in
these Quick Reference Sheets include the following:

Priority Queuing (PQ)

Custom Queuing (CQ)

Modified Deficit Round Robin (MDRR) queuing

Weighted Fair Queuing (WFQ)

Class-Based WFQ (CB-WFQ)

Low Latency Queuing (LLQ)

Compression—By compressing a packet’s header or
payload, fewer bits are sent across the link. This effectively gives you more
bandwidth.